1. Field of the Invention
The present invention relates to an actuator using a laminar piezoelectric or electrostrictive driver for operating a given driven member, and a dot-matrix wire dot print head or printer using such piezoelectric actuator for operating a print wire. More particularly, this invention is concerned with techniques for improving the durability or life expectancy of the laminar piezoelectric driver.
2. Discussion of the Prior Art
A piezoelectric driver uses a piezoelectric element consisting of a piezoelectric ceramic film or layer, and two electrode films or layers which are formed on the opposite surfaces of the piezoelectric ceramic layer. The piezoelectric ceramic layer is displaced, i.e., contracted and expanded or elongated as a voltage applied thereto is changed. It is known to use this piezoelectric element for an actuator, wherein the contraction and expansion of the piezoelectric element are utilized to operate a desired member to be driven. Since the amount of displacement of the piezoelectric element obtained by changing the applied voltage is small, a relatively large number of the piezoelectric elements are superposed on each other to provide a laminar piezoelectric driver which is capable of undergoing contraction and elongation by a large amount. In operation, the laminar piezoelectric driver is elongated (expanded) in the direction of lamination when a voltage is applied (when the applied voltage is increased) to polarize the piezoelectric ceramic layers in the direction parallel to the direction of lamination. When the applied voltage is removed or decreased, the laminar piezoelectric driver is contracted in the direction of lamination. On the other hand, the driver is contracted and elongated in the direction perpendicular to the direction of lamination (direction of polarization) when the voltage is applied and removed, respectively. In a known piezoelectric actuator, however, the contraction and elongation of the laminar piezoelectric driver in the direction of lamination are used to drive the desired driven member. Namely, the conventional laminar piezoelectric driver is fixed to a frame of the actuator, at one of opposite ends which are opposed to each other in the direction of lamination. The other end of the driver is operatively connected to a suitable transmission member or mechanism for transmitting the displacement of the driver to the desired driven member. This type of laminar piezoelectric driver is referred to as "longitudinal-effect" type. A dot-matrix wire dot print head is an example of a piezoelectric actuator which uses such a longitudinal-effect type piezoelectric driver, wherein the displacement of the laminar driver is used to operate a print wire.
However, the known piezoelectric actuator discussed above suffers from a problem. More specifically, the tensile strength of the laminar longitudinal-effect type piezoelectric driver in the direction of lamination depends on the tensile strength of the electrode layers which is relatively low, and on the bonding strength between the piezoelectric ceramic layers and electrode layers. Accordingly, the longitudinal-effect type driver is not sufficiently resistant to the tensile force applied in the direction of lamination. When the laminar driver is contracted in the direction of lamination upon removal of the applied voltage, a tensile force is exerted on the laminar driver since the driver is rapidly contracted while the transmission mechanism tends to remain due to inertia. Thus, the known laminar longitudinal-effect type piezoelectric driver has a high risk of damage due to its relatively low tensile strength.
Similarly, the bending strength of the laminar longitudinal-effect type piezoelectric driver in the direction of lamination depends on the bending strength of the electrode layers, and on the bonding strength between the ceramic and electrode layers. That is, the driver has a low bending strength in the direction of lamination, and is likely to be damaged due to a bending force applied thereto upon generation of a moment by the transmission mechanism.